1. Field of the Invention
This invention relates to signal error detecting. More particularly, but not exclusively, the invention relates to apparatus for achieving correct field framing on reproduction of a digital PAL television signal from a video tape recorder (VTR).
2. Description of the Prior Art
The color sub-carrier frequency used in a 625 line PAL television system is 4.43361875 MHz, and the use of this frequency results in a whole number of cycles of the sub-carrier frequency in every sequence of eight successive fields. When a television signal is converted to digital form, for example for recording, it is usual to code only the actual video information, and to omit the synchronizing signals and the color burst signals. When reproducing a composite coded video signal, or in some cases as decoded video signal, it is necessary to know the original eight field sequence when adding back the synchronizing signals and the color burst signals. Thus if the coded video signal comprises blocks of data comprising words corresponding respectively to a sequence of samples, it is necessary to know the phase of the color sub-carrier of the first sample in each data block.
The eight field sequence may be identified by including identifier bits in each block of data so that on reproduction a square wave eight field identifier signal can be derived from these bits. If, however, these bits are lost or corrupted, then it may not be possible to identify the eight field sequence.